As a consequence of increasingly strict legal regulations concerning permissible harmful emissions in motor vehicles which have internal combustion engines, the harmful emissions must be kept as low as possible during operation of the internal combustion engine. On one hand, this can be achieved by reducing the harmful emissions that are produced during the combustion of the air/fuel mixture in the respective cylinder of the internal combustion engine. On the other hand, exhaust gas postprocessing systems are used in internal combustion engines, converting the harmful emissions that are produced during the combustion process of the air/fuel mixture in the respective cylinder into harmless substances. Catalytic converters are used for this purpose, converting carbon monoxide, hydrocarbons and nitrogen oxide into harmless substances. Both selectively influencing the generation of harmful emissions during the combustion, and efficiently converting the harmful components by means of a catalytic converter for exhaust gases, require the air/fuel ratio in the respective cylinder to be adjusted very precisely.
In this context, it must be ensured that the components of the exhaust gas postprocessing system also function in the desired manner for a long period of operation, and that faults can be reliably detected.
NOx sensors are used for determining the nitrogen oxide content in the exhaust gas downstream of the catalytic converter.
An NOx sensor which is based on a ZrO2 ceramic and has two chambers is disclosed in the textbook entitled “Handbuch Verbrennungsmotoren”, edited by Richard von Basshuysen/Fred Schäfer, 2nd edition, Jun. 2002, published by Friedrich Vieweg & Sohn Verlagsgesellschaft mbH Braunschweig/Wiesbaden, page 589 ff. In the first chamber, a constant partial pressure of the oxygen contained in the exhaust gas is produced by applying a pump flow. The pump flow is inversely proportional to the air/fuel ratio. In the second chamber, the nitrogen oxide contained in the exhaust gas is decomposed by application of a further flow. This flow is proportional to the nitrogen oxide content in the exhaust gas and forms the measurement signal of the NOx sensor.
Components in the exhaust gas can cause contamination of the NOx sensor. This can make it necessary to carry out a diagnosis of the NOx sensor during routine operation of the internal combustion engine, wherein a check is performed under specified conditions in order to establish how the determined measurement signal of the NOx sensor correlates to the measurement signal of the NOx sensor in its new state.